Field
The disclosed concept pertains generally to vacuum circuit breakers and other types of vacuum switchgear and related components, such as vacuum interrupters and arc-resistant shields. In particular, the disclosed concept pertains to a shield structure including an arc-resistant material which is hermetically sealed to a ceramic substrate of a vacuum interrupter, such as used in a vacuum circuit breaker.
Background Information
Vacuum interrupters are typically used to interrupt high voltage AC currents. The interrupters include a generally cylindrical vacuum envelope surrounding a pair of coaxially aligned separable contact assemblies having opposing contact surfaces. The contact surfaces abut one another in a closed circuit position and are separated to open the circuit. Each electrode assembly is connected to a current carrying terminal post extending outside the vacuum envelope and connecting to an AC circuit.
An arc is typically formed between the contact surfaces when the contacts are moved apart to the open circuit position. The arcing continues until the current is interrupted. Metal from the contacts that is vaporized by the arc forms a neutral plasma during arcing and condenses back onto the contacts and also onto a vapor condensing shield placed between the contact assemblies and the vacuum envelope after the current is extinguished.
The vacuum envelope of the interrupter generally includes a ceramic tubular insulating casing with a metal end cap or seal covering each end. The electrodes of the vacuum interrupter extend through the end caps into the vacuum envelope. At least one of the end caps is rigidly connected to the electrode and must be able to withstand relatively high dynamic forces during operation of the interrupter.
Various designs of interrupters are known in the art. There are full ceramic designs wherein the tubular insulating casing is composed completely of ceramic material. There is also known a design which includes a center portion composed of a metal shield with a ceramic portion located on both ends of the metal shield. This design is commonly referred to as a “belly band” interrupter.
Vacuum interrupters are key components of vacuum-type switchgear. It is typical for interrupters for vacuum-type circuit breakers using transverse magnetic field contacts to include a vapor shield, e.g., internal arc shield or arc-resistant shield, that is resistant to heavy arcing to restrict the outward dissemination of the arc and preserve the high voltage withstand of the interrupter after breaking the fault current.
It is customary for the shield to be constructed of copper, stainless steel, copper-chromium alloy or a combination thereof. In some cases, the shield may be constructed of one material in the arcing area and a second material may be used for the remainder of the shield. The copper-chromium alloy material may be used for the highest fault current ratings because of its resistance to arc damage and its ability to hold off high voltages after the arcing has occurred. It is typical for the copper-chromium alloy to include about 10 to 25% by weight chromium and the balance copper.
It is an object of the disclosed concept to develop new arc shield designs, for example, which can accommodate large contacts employed to interrupt large currents. Further, it is an object of the invention to design an arc shield that can be hermetically sealed to a ceramic insulator positioned at both ends of a vacuum interrupter. It is believed that such positioning of the shield provides available space for large contacts to be used as compared to an arc shield mounted fully inside of a fully ceramic insulating casing of a vacuum interrupter.